Bridget Jones, you could argue, was the first truly modern comic heroine. Back in the mid-90s, through Bridget and her friends, Helen Fielding identified the confusion of a new generation of women and – crucially – allowed her readers to laugh at it. The books went on to sell 15m copies in 40 countries, were adapted into two hit films and turned their heroine into shorthand for a particular type of contemporary womanhood.

Now the author has revived Bridget, nearly 20 years on, to negotiate a whole new sexual and cultural landscape. The girlish style has not changed, despite being 51: Bridget still obsessively logs her weight, her alcohol units and pieces of Nicorette gum (she's given up the Silk Cut); to this litany of guilt she can now add embarrassing texts, tweets and Botox.

At times this tone (Gah! Hurrah! V v good, etc) makes her sound annoyingly like a giddy teen, as it always did. But there is a shadow over this new instalment: Bridget is now a widow and mother of two small children. Despite the distress of many fans, it's a brilliant solution to the obvious problem of a third book. Bridget's raison d'etre is the quest for a man, so the happy ending she found in The Edge of Reason must be reversed, returning her to her natural state of hapless relationships and self-help books. By making her a widow, Fielding allows Mark Darcy to remain as implausibly perfect in death as he was in life (killed by a landmine while negotiating the release of aid workers in Sudan, no less), avoiding any tarnishing of the dream with a messy divorce and offering plenty of scope for tear-jerking moments with the children. But she uses these darker notes sparingly; Bridget's very British determination to "Keep Buggering On", as she puts it, nudges the tragedy to the periphery most of the time, but it does give the character a poignancy she lacked before.

Mad About the Boy begins four years after Mark's death, as Bridget emerges from the first raw shock of grief to engage with the dating scene again. And how different that scene looks now – when she was last single there was no Twitter, sexting or online dating, and a cougar was just a big cat. Fielding enjoys milking all of these for comic possibilities, though Bridget's being such a technological late adopter makes a lot of the observational comedy sound dated. She's on surer ground when it comes to slapstick and there are some lovely set pieces based on misunderstandings and bad timing – usually when Bridget happens to run into her son's disapproving (yet ruggedly handsome) teacher, Mr Wallaker.

As if to compensate for the cruel blow she has dealt Bridget, Fielding has made her leading men even more idealised here, but that has always been part of the character's appeal – the ordinary heroine who wins her romantic hero not by being the perfect woman, but by being her clumsy self.

And why not? It's fiction. I've always been surprised at how furiously some women work themselves up over Bridget in the name of feminism. She's not Minister for Women, she's just a character in a romantic comedy, a genre that has always demanded resolution in the form of lovers uniting. In reviving Bridget now, Fielding has dared to question the happy ending, and in doing so she holds a mirror up to our changing values. True love is not guaranteed for life, even in romcoms, and women in their 40s and 50s are no longer prepared to fade away, alone and invisible. Bridget chronicles all this in her own inimitable voice; she is supposed to be ridiculous and often infuriating. But she is also very human, with all her insecurities, and if you don't shed a few tears in the course of this book, you must have a heart of ice. In the end, though, it's hard not to feel that Fielding is hampered by her own legacy. Bridget has spawned so many imitators in the intervening years that all this ground feels very well trodden. Even so, those of us who loved her the first time will be glad to welcome her back – big pants, fillers and all.